Choose a brown you both love for sofa/sectional...whatever your needs. Put that color or color family over the chairs rail and white under.

Add lots of whites in accessories, keeping sofa/wall brown in the mix.

Without knowing your specific style or the size/view of entire room, it‘s hard to suggest specifics, ie: size/type of furniture, placement, decor, etc.

Just based on what you mentioned (white components and leather), here are general thoughts.

I’d remove chair rail and paint room all the same color, so it’s not as broken up. Suggest a warm white and your options will open up big time.

Not sure if you wanted a wall to wall unit (if you do, you may be limiting seating configurations). Try Ikea and they will design something for your needs. Also consider something less bulky. When accessorizing, add wood, baskets, (natural and warm items) to pull in the leather.

In regards to seating, a light couch with great cushions and for your husband a comfy leather chair and suggest rust instead of brown, ie: room below (but with a leather chair). You may want to do an on line room planner as a first step and when purchasing, consider flexibility and especially not a large sectional.

If liking the above, see below for an overall concept/pulling room together. Highly suggest doing, as it will be easier and will end up with a well coordinated room.

If you wanted a leather couch, here are looks that would suit above concept.